Those two terms are ambiguous. Enamel can also be acrylic. Acrylic enamel, acrylic laquer, acrylic urethane are different paint types.
The only reliable way to tell is to destroy a small area. Laquer will disolve with laquer thinner, no matter how old it is. Enamel usually melts with brake fluid but it may take some time. Urethane is impervious to just about anything.
Laquers and urethanes are safe to paint over. Enamels can be funky, I've read of people having lifting problems even with fully cured enamel. There are primers however that are safe over anything. Once you have a good primer down you can use the paint of your choice. Ask the guy at the paint store.